How to set a default server on the Speedtouch 546?

I have just upgraded from the Speedtouch 510 to the 546. I have a server on my network and for the 510 I could define the IP address of the default server on the NAPT web configuration page (DHCP is disabled). I can not seem to do this with the 546. If I go to the 'Assign the public IP address of a connection to a device' I can not seem to edit the page (under device I get 'Not assigned and no compatible device found'). How do I set up a default server on the 546?

On a separate note, if I have DHCP on, is it possible to provide a specific IP address to a specific computer? I know that the Apple Airport allows DHCP reservation lists.

Thanks

Tony